1. Field of the Invention
The present invention relates to a procedure to polarize at least one zone of a foil of ferroelectric material. In addition, the invention relates to a procedure to produce a piezoelectric or pyroelectric transducer by means of this polarization procedure.
2. Description of the Prior Art
In his preceding French patent application FR-A-2 538 157, the applicant described a procedure and a device to polarize ferroelectric materials. According to this procedure a low-frequency alternating electric field is applied to the ferroelectric material, and then the amplitude of this alternating electric field is progressively increased so that there is a controlled remaining polarization left in the material. The maximum amplitude of the electric field is relatively high because it must exceed the coercive field of the ferroelectric material.
Today this procedure is widely applied, especially to produce piezoelectric or pyroelectric transducers. In general the procedure starts with the deposition of the measurement electrodes of the transducer on the two opposite surfaces of a foil of ferroelectric material, for example by sputtering or evaporation. These electrodes are used to apply the alternating electric field during the polarization phase. The foil of ferroelectric material is generally compressed to limit volume variations which may locally occur due to the higher values of the electric field during the polarization phase, because these variations tend to adversely affect the reproducibility of the transducer's characteristics. In practice, the pressure which can thus be exerted on the foil of ferroelectric material is limited, typically to some hundred bars, otherwise there would be a deterioration of the ferroelectric material near the edges of the deposited electrodes or a deterioration of the electrodes themselves if they cover a relatively large area.
In addition, the respective parts of the measurement electrodes which extend from the polarized zone in order to provide connections must be relatively far apart from each other and parallel to the foil of ferroelectric material. This spacing is necessary because of the high voltages involved: in case of a small spacing there is the risk of breakdown, flashover or polarization of the material. As a result the dimensions of the pickup transducer increase.
It is difficult to use the procedure described above to polarize a zone with reduced dimensions on a foil of ferroelectric material, typically a zone with dimensions below 1 mm.sup.2 on a foil having a thickness of approximately 25 .mu.m. Because of these small dimensions the edge effects become significant and even preponderant and complicate the interpretation of the current measurements. These edge effects also affect the homogeneity of the induced remaining polarization. The polarization is equal to the ratio of the quantity of charges and the geometrical surface. In addition, the local stress within the ferroelectric material resulting from these small dimensions becomes important so that a higher pressure is required to limit its effect, and this results in problems of mechanical strength which have been mentioned before.
These inconveniences can also be found in connection with large surfaces, even if their impact on the measurements is less important.